The Russian companies NPK- SPP (Research and Production Company- Precision Systems and Instruments) of Moscow together with RSC (Rocket and Space Corporation)Energia of Korolev,Moscow region (also referred to as: S.P. Korolev Rocket and Space Corporation Energia), designed and developed BTLS. The overall objective is the demonstration of a laser communication experiment on the ISS-RS (Russian Segment).

BLTS is comprised of two modules, BTLS-N and BTLS-V. The BTLS-N module is installed on the outside of the MLM (Multipurpose Laboratory Module, Nauka) of the ISS-RS (Russian Segment), while the BTLS-V instrumentation is kept inside the MLM. 1)2)

Launch: In August 2011, BTLS-N was delivered to the ISS by a Progress cargo spacecraft.

Orbit of the ISS: Near-circular orbit in the altitude range of 350-450 km, inclination = 51.6º.

The space experiments with BTLS were carried out on a Russian segment of the ISS in the timeframe 2011-2013. 3)4)

• The BTLS-N terminal was installed on the outer surface of Russian ISS segment on August 3, 2011 (Figure 3). Afterwards, the first on-board hardware tests were done for the entire systems and the temperature control modes.

• The first targeting system workout was done on August 30, 2011. During the next demonstration sessions, different algorithms of BTLS with respect to ground-station targeting were tested.

• During the sessions on October 5 and -7, 2011, the current protection system determined an error in the power supply from the ISS. Hence, the following sessions were delayed. After an analysis of possible error reasons, a device for current control was developed and delivered to the ISS in August 2012. After successful device tests, the BTLS experiments were continued. In September 2012, the BTLS targeting experiments were repeated.

• The first information transfer session was carried out on September 30, 2012. A test image was transferred with 125 Mbit/s baud rate. Next, the target scientific information was transferred from the ISS-RS board with a baud rate of 125 Mbit/s.

• In July 2013, BTLS test information was transferred with a baud rate of 622 Mbit/s. At this point all goals of experiment program were achieved.

BTLS-NLT-1 communication line range

≤1000 km

Test information transfer bandwidth

3 or 125 or 622 Mbit/s

Data transfer bandwidth

125 Mbit/s

Communication session time

≤ 5 minutes

Information transceiver wavelength

1.55 µm

Information transceiver power

6 W

Information receiver wavelength

0.85 µm

Targeting system reference marks wavelength

0.81 µm

Targeting system receivers wavelength,

0.78 µm

Information transceiver beam divergence on -3 dB

60 arcsec

Reference mark beam divergence on -3 dB

3.3º

Target lock detector field of view

35 arcmin

Tracking detector field of view

4.2 arcmin

Search and target lock detector field of view

2º

Information receiver field of view

3.5º

Instrument power consumption

≤300 W

Table 1: Technical parameters of BTLS-N

Figure 3: Illustration of the BLTS-N module on the outside of the MLM (Multipurpose Laboratory Module, Nauka) of the ISS-RS (image credit: NPK-SPP)

Ground laser terminal NLT-1

Experiments were conducted on the BTLS-NLT-1 communication link at data rates of 3, 125 and 622 Mbit/s.

For communication with the on-board terminal and information transfer, a ground terminal, NLT-1, was developed. This terminal was installed on the “SON Arkhyz” optical observing station, located in the North Caucasus. The optical-mechanical module of NLT-1 in the dome of the ground station is shown in Figure 4. The power, communication and control modules are installed in separate building.

After the BTLS installation on ISS, board test sessions were performed to check the work capabilities; also the temperature measurement of the critical BTLS-N functions was done taking the sunlight into account. The temperature measurement has shown that the temperature corresponds to previously done calculations.

The next part of the experiments was devoted to targeting of the on-ground and on-board terminals. The targeting task in this experiment is fairly complicated due to the large size of the ISS and the low precision of its trajectory prediction. It affects mainly the arrival time of the ISS. Without knowing the precise location of the BTLS-N terminal on the ISS, and it’s coordinate system shift, it’s hard to calculate the target coordinates (mount of the rotation angles) for the onboard terminal, using only the information of the ISS ephemeris.

To compensate for these errors during the startup phase of the targeting experiments, a BTLS vision line scanning mode was implemented. It's purpose is to locate the on-ground laser reference marks; after this completion, the BTLS should lock onto the target and start it’s auto-tracking.

The red line of Figure 5 shows the trajectory of the BTLS mount vision line movement during a scan and after an on-ground reference mark lock. The blue line is the signal of the on-ground detector camera.

In the first experimental sessions of the on-ground terminal targeting, and further ISS tracking at the instant of signal appearance from the on-board laser reference, markings onto the array detector were performed in the zone of the ISS pass through the wide-field detector, using the reflected of spacecraft sunlight. In further sessions, the targeting was performed in “blind” mode using the ISS ephemeris and additional time error compensation with a lock on further on-board laser marks.

The results of the mutual targeting of the on-board and on-ground terminals are shown in Figure 5. One can see the trajectory convergence of the BTLS-N vision line movement (red line), the video frames from the NLT-1 array detector and the signal brightness of the ISS radiation on this detector (blue line). Also, one can notice the trajectory of the BTLS vision line scanning, and splashes of NLT-1 detector signal brightness in instances, when the vision line passed through its field of view.

At the trajectory red arrow mark in Figure 5, the lock of the on-ground reference mark by the on-board terminal was achieved and tracking started. During this time vision line has passed twice through the clouds. Signal brightness of the BTLS reference mark in these times was decreased to the level of the background, but due to the predicted movement, the tracking was not lost in the on-board and the on-ground equipment. The tracking error was less than 1 arcsecond.

After a successful system mutual targeting session, 3 types of information transfer sessions were performed. Initially, the test information (an image of ISS, stored in the on-board computer of BTLS-N), was transferred with a baud rate of 125 Mbit/s. Figure 6 illustrates, how the quality of transfer changes during a tracking session. The changes are due to the elevation angle between NTL-1 and BTLS-N , which is decreasing from a maximum of 30º to 13º above the horizon. On low angles, the influence of atmospheric distortion on the data transfer gets markedly worse. Also, the test information on the 622 Mbit/s baud rate was transferred during this part of the experiment.

Figure 6: Targeting system experimental results; at left is the ISS image at 30º, at right is the image of 15º elevation (image credit: NPK-SPP)

The science data information transfer from the ISS on-board BTLS-V during the session was performed with a data rate of 125 Mbit/s. The error range during the transfer increased from 3 x10-8 up to 10-6. The atmospheric influence on the transferred information quality was not evaluated in this part of the work.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).